Nitrogen oxides such as NO and NO2, referred to collectively as NOx, are common constituents of emissions in the exhaust gas of diesel engines. The levels of these pollutants are controlled to meet emissions standards by reducing them to nitrogen gas at a selective catalytic reduction catalyst (SCR catalyst) that uses injected urea or ammonia as a reductant. However, due to the prolonged time required to heat up exhaust after-treatment devices and achieve catalytic light-off, such as during cold starts, light acceleration and low speed-load cruises, NOx emissions from engine-out cold starts can contribute a significant fraction of the total NOx emissions.
There are several approaches to address this issue. One example approach shown in U.S. Pat. No. 8,407,987 by Andersson discloses a control method for an exhaust after-treatment system of an engine in which the flow of the components of the exhaust gas are oxidized in an oxidation catalyst, and then reduced in a SCR catalyst. The exhaust flow through the oxidation catalyst is controlled depending on a desired ratio among the exhaust constituents, which is based on a temperature of the SCR catalyst that maximizes selected chemical reactions.
However, the inventors have identified potential issues with such an approach. As an example, while the method of Andersson adjusts a NOx ratio using an oxidative catalyst, the oxidative catalyst does not store NOx. As a result, Andersson relies on the regulation of exhaust flow away from, or over, the oxidative material via valves to control how much NOx is available at the oxidative catalyst at any given time. As such, this configuration may be inefficient during relatively cool operating temperatures in optimally facilitating conversion of NOx. This is due to the inability of the reduction catalyst to reach its light-off temperature before the oxidized exhaust gas contacts the reducing agent. Consequently, the NOx may slip through the SCR catalyst without being chemically converted into N2 and N2O.
The inventors herein have recognized the above issue and identified an approach to at least partly address the issue. In one example approach, a method for controlling NOx levels in the feedgas of an engine having a passive NOx adsorber (PNA) and a SCR catalyst in the exhaust passage is provided. The method comprises: adjusting one of a fuel injection timing and an EGR rate based on the storage on and release of NOx from a passive NOx adsorber (PNA) to maintain a NOx species ratio upstream of an exhaust SCR catalyst in an exhaust after-treatment device. In this way, conversion of NOx into non-polluting forms, such as N2, is facilitated, thereby reducing vehicle emissions.
In one example, an exhaust system may include a PNA positioned in an exhaust passage upstream of a SCR catalyst. During an engine cold-start condition, NOx emitted from the engine, in the form of NO, is stored on the PNA until the PNA reaches a pre-determined temperature above the SCR catalyst light-off temperature. More specifically, after adsorbing NO, the PNA may oxidize the NO such that the primary species is NO2, stored as nitrates, and these nitrates decompose at a temperature above the SCR catalyst light-off temperature to release the NO2 into the exhaust. Based on the amount of NO emitted from the engine, and further based on whether the PNA is storing NOx or releasing NO2, a ratio of NOx species downstream of the PNA and upstream of the SCR catalyst may vary. In particular, based on the loading and release of NOx onto and from the PNA, an amount of NO from the engine may pass through the PNA without being converted to NO2. As elaborated herein with reference to FIG. 3, during conditions when NO2 is being released from the PNA, an EGR rate may be reduced or increased and/or fuel injection timing may be advanced or retarded so as to increase or decrease a concentration of NO downstream of the PNA and upstream of the SCR catalyst. Consequently, by adjusting the EGR rate and/or the fuel injection timing, a selected NOx species ratio may be maintained upstream of the SCR catalyst. The selected ratio may correspond to a specific ratio of NOx species (such as a specific ratio of NO to NO2) that allows for maximal conversion of NOx to N2 by the reductant and SCR catalyst. The adjustment to EGR rate and fuel injection timing may be based on an estimation of NOx stored on and released by the PNA, as determined based on operating condition and exhaust gas measurement output by NOx sensors disposed before and after the PNA. In some embodiments with a single NOx sensor after the PNA (i.e. no pre-PNA NOx sensor), the pre-PNA NOx concentration is estimated from engine conditions, including speed, load, EGR setting, fuel injection timing, etc.
In this way, by adjusting the EGR rate and the fuel injection timing, a pre-determined ratio of NO to NO2 species is achieved upstream of the reductant injector and SCR catalyst and downstream of the PNA. This allows NOx conversion to be improved during engine cold-starts. By storing NO at a PNA and then releasing NO2 from the PNA at a pre-determined temperature above the light-off temperature of a downstream SCR catalyst, a controlled discharge of NOx is enabled. In particular, NOx is discharged from the PNA only when optimal conditions are met to reduce inefficient catalytic conversion of NOx to N2. Thus, it is possible to substantially reduce release of NOx species in vehicle emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.